x86/tsc: Move sync cleanup to a safe place

Cleaning up the stop marker on the control CPU is wrong when we want to add
retry support. Move the cleanup to the starting CPU.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/20161119134017.892095627@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/tsc_sync.c

@@ -266,11 +266,6 @@ void check_tsc_sync_source(int cpu)
 		return;
 	}
 
-	/*
-	 * Reset it - in case this is a second bootup:
-	 */
-	atomic_set(&stop_count, 0);
-
 	/*
 	 * Wait for the target to start or to skip the test:
 	 */
@@ -360,6 +355,11 @@ void check_tsc_sync_target(void)
 	 */
 	while (atomic_read(&stop_count) != cpus)
 		cpu_relax();
+
+	/*
+	 * Reset it for the next sync test:
+	 */
+	atomic_set(&stop_count, 0);
 }
 
 #endif /* CONFIG_SMP */